High sensitivity non-destructive measurement techniques are required for process control in the fabrication of electronic devices. In order to attain the earliest possible feedback during production, it is necessary to characterize electronic properties before the device is complete. Importantly, the physical phenomena which governs device operation occurs in ultra thin active layers which are difficult to characterize due to their small volume. For example, advanced transistor structures may comprise a thin strained silicon layer, wherein the electrical properties of the transistor are controlled by straining of the silicon lattice. Conventional metrology techniques such as ellipsometry cannot effectively characterize the electronic properties of such thin films. Fortunately, an optical technique known as photo-reflectance may be used to characterize the electronic properties of thin films. The conventional photo-reflectance configuration employs an amplitude modulated laser pump beam to induce small periodic changes in electron-hole population in the thin film of interest. A second optical beam, coincident with the modulated pump beam is then used to monitor small sample reflectivity changes using phase locked detection. This disclosure describes the application of a new photo-reflectance metrology technique to characterize active electronic properties of nanometer thickness silicon films.
The method of photo-reflectance characterization of strain and active dopant in semiconductor structures disclosed herein attains sensitivity to electronic properties of Si nanofilmstructures by using a probe wavelength which is near the first strong interband transition energy in Si, which occurs at a wavelength of approximately 375 nm. In the vicinity of such a transition the photo-reflectance (PR) signal typically will exhibit a sharp derivative-like shape. Generally, the PR signal takes the form ΔR/R=αΔ∈1+βΔ∈2, where α and β are the “Seraphin coefficients” which contain filmstack information, and Δ∈1 and Δ∈2, are the pump induced changes in the real and imaginary parts of the dielectric function, respectively (Seraphin, 1965). In other words, Δ∈1 and Δ∈2 describe the pump induced modulation of thin film properties. These induced changes may be written as the product of the energy of the free carrier and a third derivative of the semiconductor dielectric function as follows: Δ∈i=∂3(ω∈i)/∂ω3×UP, where UP is the free carrier energy and ω is the photon frequency (Aspnes, 1980). Thus, the motivation for choosing the wavelength of the probe beam at 375 nm for Si lies in the sharp derivative form for Δ∈1 and Δ∈2. This third derivative term may be calculated directly from known semiconductor optical constants. The total PR signal therefore becomes ΔR/R=Re[(α−iβ)×∂3(ω∈)/∂ω3)]×UP. The third derivative functional form is large only nearby strong optical absorptions in the semiconductor band structure, and thus may isolate these features with great precision. This is what allows the PR technique to precisely measure strain in nanoscale strained silicon layers, for example, since the strong optical absorption in Si near 375 nm undergoes a precise shift under strain. Nearby to these strong optical absorptions, the amplitude of the PR response also has excellent sensitivity to electric fields in activated silicon transistor channel regions: note he free electron energy is given by the expression UP=e2h2F2/24mω2, where e is the electronic charge, h is Plank's constant, F is the space charge field, and m is the electronic effective mass. This free electron energy is also proportional to the induced carrier density, which may be seen from the Poisson relation: Ne=∈oF2/2 eV, where Ne is the induced carrier density, V is the built-in surface voltage and ∈o is the permittivity of the material (Shen, 1990).
A primary problem with common commercial photo-reflectometers is the wavelength of the probe beam is not selected to coincide with strong optical absorptions in the electronic material under investigation (Salnick, 2003; Borden, 2000). Thus, in conventional photo-reflectometers, the PR signal is obtained at wavelengths where the third derivative of the dielectric function is small and therefore no information about band structure is available. Thus, conventional photo-reflectometers cannot usefully determine internal electric fields or strain. Rather, these photo-reflectometers are sensitive to the damage profile of implanted dopant (Salnick, 2003). This filmstack information contained in the PR signal is of secondary importance, and produces a cosine-like curve in the PR response as a function of implant depth. Furthermore, implant depth dependence cannot be decoupled from the implant dose dependence in these conventional photo-reflectometers. In any event, the filmstack information provided by conventional commercial photo-reflectometers is available through standard linear optical techniques such as spectroscopic ellipsometry (Jellison, 1995).
A further problem with conventional photo-reflectometers that do employ a lamp based spectroscopic probe beam with wavelengths in the vicinity of strong optical transitions, is that when using such a beam, they must either i) use a monochrometer for sequential phase locked measurements at each desired wavelength, or ii) use multiple phase locked detection circuits operating in parallel with a linear photodiode detection array. In the case of use of a monochrometer, the total single point measurement time is typically on the order of 5-10 minutes, which is not satisfactory for use in volume manufacturing. In the case of use of parallel phase locked circuits, the cost and complexity of the apparatus are maximized. Moreover, in conventional photo-reflectometers that employ such a lamp based spectroscopic probe beam, the lamp provides incoherent light and hence cannot be focused to a small spot as effectively as a laser beam. In the method of photo-reflectance characterization of strain and active dopant in semiconductor structures disclosed herein all of these problems are solved in an elegant manner. First, the use of a monochrometer is unnecessary because the laser probe wavelength is either preset at a known wavelength of interest, or is rapidly scanned over a multiplicity of such known wavelengths. Second, parallel phase locked circuits are unnecessary because only one detection photodiode is required. And finally, the use of a laser source allows tight focusing and rapid data acquisition in accord with process control requirements for volume manufacturing.
An additional problem with common commercial photo-reflectometers is the wavelength of the pump beam is not selected to provide an absorption depth suitable for effective pumping of insulating substrates commonly used in semiconductor manufacturing. For example, in order to effectively pump silicon-on-insulator substrates, the pump laser wavelength is constrained by the requirement the absorption depth be less than or commensurate with the top silicon thickness. This implies suitable pump wavelengths of less than approximately 500 nm, a condition which is not satisfied by common commercial photo-reflectometers (Salnick, 2003).
Thus, while conventional photo-reflectometers/spectrometers may be suitable for the particular purpose to which they address, they are not as suitable as is this disclosure for the characterization of active electronic properties of semiconductor nanostructures before the device is complete.
In these respects, the method of photo-reflectance characterization of strain and active dopant in semiconductor structures disclosed herein substantially departs from the conventional concepts and designs of the prior art, and in so doing, provides an apparatus primarily developed for the rapid characterization of active electronic properties of semiconductor nanostructures in volume manufacturing.